The National Oceanic and Atmospheric Administration’s (NOAA) Warn-on-Forecast (WoF) program aims to provide frequently updating, probabilistic model guidance that will enable National Weather Service (NWS) forecasters to produce more continuous communication of hazardous weather threats, including heavy rainfall and flash floods. To evaluate the application of the WoF concept for probabilistic short-term flash flood prediction, the 0-3 hour rainfall forecasts from NOAA’s National Severe Storms Laboratory’s (NSSL) experimental WoF System (WoFS) were integrated as the forcing to the NWS operational hydrologic modeling core within the Flooded Locations and Simulated Hydrographs (FLASH) system.
Now, this might seem counterintuitive…
Coupled Hydro-Meteorological Modelling
The successful evaluation of the deterministic flash flood products from the FLASH system during previous Hydrometeorology Testbed experiments resulted in the transition of the FLASH suite to the National Centers for Environmental Prediction (NCEP) in 2018. The FLASH products are now used routinely in NWS weather forecast offices and at the NWS Weather Prediction Center for flash flood monitoring, detection, and decision making.
However, the operational FLASH products neither provide probabilistic information to communicate the uncertainty associated with the forecasts, nor do they provide significantly longer flash flood forecast lead time. To extend the hydrometeorological forecast lead time beyond the watershed response time, it is crucial to explore the use of short-term quantitative precipitation forecasts (QPFs) from numerical weather prediction (NWP) models as forcing for the hydrologic model.
The QPFs and probabilistic QPFs (PQPFs) from NSSL’s experimental WoFS can be used as a forcing pathway for extending lead times in flash flood forecasting. The potential application of WoFS 0-3 hour ensemble QPFs for flash flood prediction was analyzed during the 2018 Hydrometeorology Testbed – Multi-Radar Multi-Sensor (HMT-Hydro) experiment held in Norman, Oklahoma. This experiment was the first attempt to couple an atmospheric and a hydrologic ensemble system for probabilistic flash flood forecasts at the storm scale.
Ensemble Prediction Systems
The experimental WoFS used in this study is a 36-member Advanced Research version of the Weather Research and Forecasting (WRF-ARW) Model-based ensemble data assimilation and prediction system. The WoFS domain covered a ~900-km wide region at 3-km horizontal grid spacing and was centered over the region where the hazardous weather was anticipated.
The WoFS used the experimental High-Resolution Rapid Refresh (HRRRE) ensemble developed by NOAA’s Earth System Research Laboratory’s Global System Division for initial and boundary conditions. Once initialized from the HRRRE, the WoFS was cycled every 15 minutes to assimilate all available storm observations into the system, including MRMS radar reflectivity and Level II radial velocity data, cloud water path retrievals from satellite, and conventional observations.
The flash flood forecast cycling for all experiments mimicked that of the operational FLASH system: a new hydrologic forecast out to 12 hours at a 5-minute time step was launched every 10 minutes and produced maxima of streamflow, unit streamflow, and soil saturation. The resulting deterministic products from the QPE-forced FLASH system are input to a postprocessing algorithm based on the statistical analysis of conditional distributions of measured streamflow to generate the probabilistic FLASH forecasts.
For the condition using the WoFS QPFs, all 36 ensemble member WoFS QPFs are ingested in the hydrologic model, which results in 36 forecasts of maximum unit streamflow. Each of these 36 maximum unit streamflow forecasts goes into the postprocessing algorithm to produce a singular mean output of the different probabilistic FLASH products.
Experimental Design and Evaluation
Nine NWS forecasters evaluated a series of QPE comparison products as well as gridded deterministic and probabilistic flash flood products from the FLASH hydrologic system for three archived case studies. The baseline products consisted of MRMS reflectivity, radar-only QPE, and QPE comparisons such as the QPE-to-FFG (flash flood guidance) ratio and QPE average recurrence interval.
The experimental probabilistic products included the probability of receiving a flash flood report and the probability of exceeding maximum unit streamflow values to determine minor, moderate, and major flash flooding potentials. These products were used to compare the QPE-forced FLASH to the WoFS-FLASH system.
The testbed participants assessed flash flood threats for both deterministic and probabilistic flash flood products using three data conditions: 1) deterministic flash flood products from QPE-forced FLASH system, 2) probabilistic flash flood products from QPE-forced FLASH, and 3) probabilistic flash flood guidance from WoFS-FLASH. Comparisons between participants’ assessments of the flash flood threat for the three data conditions were analyzed to understand how each data condition contributed to participants’ interpretation and expectations for flash flooding events.
Key Findings and Implications
The participants’ evaluations of the coupled WoFS and hydrologic modeling system show promise of WoFS to enhance the decision-making process for flash flood events. Participants’ threat assessment and monitoring phases started earlier with the addition of WoFS forcing (condition 3) compared to the QPE-only forcing (condition 2). In several instances, participants reported expected actions of warning issuance 1-3 hours earlier when using the WoFS-FLASH guidance (condition 3) compared to the deterministic and QPE-only forcing guidance (conditions 1 and 2).
The participants’ expected actions also suggest that the WoFS-FLASH probabilistic products will result in earlier communication of flash flood threats to the public and partners. These findings indicate that WoFS-FLASH guidance can improve decision making during real-time flash flood events, and motivate a subsequent study to simulate real-time flash flood warning operations to examine measurable impacts on forecasters’ decisions.
However, some participants did not issue warnings earlier due to a lack of confidence in the WoFS-FLASH system, not believing the probability values were indicative of a flash flooding threat, or being uncomfortable issuing warnings before rain fell in the forecasted threat area. Feedback from the participants also reveals biases in the experimental probability products, where the magnitude of the Prob_LSR (probability of receiving a local storm report) products were generally perceived as higher than expected, while the Prob_USF_Minor (probability of exceeding 2 m3/s/km2 unit streamflow) products were perceived as being too low.
These findings highlight the need to demonstrate numerous aspects of the coupled hydro-meteorological modeling system prior to forecasters establishing trust with the output and using it to make real-life actionable decisions. Understanding how the probabilities are calculated, and how they compare across products, will be important for ensuring that forecasters can effectively interpret and apply them during their decision-making processes.
The lessons learned from the HMT-Hydro experiment provide a pathway to advance the science and application of WoFS PQPFs for operational hydrologic forecasts of flash flooding. Achieving progress in these areas will require not only basic research, but also collaborations between researchers, practitioners, emergency managers, and the public. By coupling advanced atmospheric and hydrologic modeling systems, the National Weather Service can continue to improve its ability to provide earlier and more accurate flash flood forecasts to safeguard communities across the United States.
Example: Manchester Advanced Flood Control Project 2024
